Avian Heart Size and Cardiovascular Performance

Abstract

THE lack of adequate criteria for the interpretation of heart size in birds is apparent in the published attempts to judge "adaptations" of heart size. The pioneer work of Clark (1927) and Brody (1945), and the standardization of technique developed by Norris and Williamson (1955) and its subsequent use by Hartman (1955, 1961) and others, have produced a large number of data on heart and body weight in a wide array of avian forms; however, these data have been incompletely analyzed. It is the purpose of this paper to rework the available information in order to establish a reliable mathematical relationship between heart weight and body weight. This relationship will subsequently be used as a basis for discussion of several aspects of avian circulation. RESULTS Heart weight.-Data on heart and body weight from most sources were used directly. Heart weights were calculated from Hartman's (1955) data on heart ratio and body weight for 67 taxonomically diverse species where sample size was five or more individuals. The curve (Figure i) that relates heart weight (W•) to body weight (Wb) in birds that range in weight from 40 to 5,400 g (figured by plotting the data logarithmically and fitting a line by the method of least squares) is described by the equations: Wh = 0.045 (Wb) •.0s• (1) and log W•, =-2.653 + 1.084 log Wb (la) All weights are in grams. The 90 per cent confidence limits for the slope of the line (1.084) are 0.889 and 1.279. Although Hartman gives no value, a line which I fitted by eye to his data has a slope of approximately i, a value to be expected if heart weight is a direct function of body weight. This value and those of Clark (0.90) and Brody (0.921) fall within the 90 per cent limits for the slope given by equation (la), and therefore are considered as not statistically different. Heart weights computed from either Clark's or Brody's formula fit Hart-man's data rather well. Thus it appears to make little difference whether the heart weight of birds is expressed as the 0.9 or 1.0 power of the body weight. Because there is some evidence that the smaller birds, particularly passerines, may deviate from the metabolic patterns of larger birds (King and Farner, 1961; Lasiewski et al., 1964) a separate regression function was determined for 36 species with body weights of less than i00 g (lower 266 The Auk, 83: 266-273. April, 1966